Such a braking gap adjustor system includes an adjusting mechanism wherein an adjusting gear is engaged at its shank portion with the end of the adjusting bolt axially inserted through the cam member in such a manner that it is axially slidable and rotatable in unison with the adjusting bolt. When linings wear away, the adjusting gear, which is kept from turning in the actuation direction of the cam member by a stopper lever, is engaged in another place with the adjusting lever, which turns with the cam member in the actuation direction thereof. As a result, threaded movement of the adjusting bolt takes place, thereby to cause the cam collar to move in the axial direction and the expansion direction of the brake shoes, so that the brake shoes move away from each other for the adjustment of the braking gap.
With such an adjusting mechanism, when a load exceeding a predetermined value is applied to the stopper lever in the actuation direction, there is a variation in the position at which the adjusting lever mates with the adjusting gear, which may cause over-adjustment. Furthermore, when the stopper lever receives an overload after full-adjustment, it may possibly fail or break.